Active Antenna Systems (AAS) is an important part of the evolution of the Long Term Evolution (LTE) and an essential part of the Fifth Generation (5G) mobile communication. AAS is a generic term that is often used to describe radio base stations that incorporate a large number of separate transmitters and antenna elements that can be used for Multiple Input Multiple Output (MIMO) and beamforming as an integrated product. MIMO enables multiplying of the capacity of a radio link by using multiple input antennas and multiple output antennas. Beamforming can be described as a signal processing technique used for controlling the directionality of transmission and reception of radio signals. 3GPP defines an AAS base station as a “BS system which combines an Antenna Array with an Active Transceiver Unit Array and a Radio Distribution Network” (3GPP TS 37.105 V13.0.0 (2016 March), Release 13). According to the 3GPP, an AAS has a radiation pattern which may be dynamically adjustable. In a “normal” base station, i.e. a base station which is a non-AAS base station, the radio equipment and the antenna are separated. In addition, a normal base station does not have capability of advanced antenna features such as an AAS base station.
AAS will be one of the key aspects of 5G as the industry moves higher up in frequency and more complex array antenna geometries are needed to achieve the desired link budget. At higher frequencies (e.g. 15 GHz, 28 GHz or higher), propagation losses are much greater than in currently used frequency bands (e.g. around 1-2 GHz). Furthermore, it is envisaged that base station transmissions will take place within higher bands in the microwave and millimeter-wave region. Since the transmit power of both base stations and user equipment is limited by physical constraints and considerations such as Electro-Magnetic Fields (EMF) for base stations and Specific Absorption Rate (SAR) for user equipment, it is not possible to compensate the increased penetration losses and provide sufficient Signal-to-Interference+Noise Ratio (SINR) within wider bandwidths (wider than the normal bandwidths which are around e.g. 1-20 MHz) simply with increased transmit power. In order to achieve the link budgets required for high data rates (e.g. 1 Gbit/s), beamforming will be necessary. It is therefore expected that integrated active arrays will become a mainstream base station building practice in the 5G era.
It is envisaged that New Radio (NR) and 5G will operate in higher frequency bands than today. For example, 4 Giga Hertz (GHz) is discussed for first systems in Japan, whilst the World Radio Communication Conference 2019 (WRC19) may allocate spectrum up to 6 GHz. Further into the future, it is envisaged that the International Telecommunication Union (ITU) and/or regional regulators may allocate microwave and millimeter wave spectrum in the range 10-100 GHz.
Antennas, base stations, AAS etc. may be tested to ensure that they meet specifications or simply to characterize it. Parameters that may be measured during testing may be for example transmit power, radiated unwanted emission, antenna gain, radiation pattern, beam width, polarization, impedance etc. A test may be performed in different ranges, such as far-field range, near-field range, free-space range, etc.
In a far-field test range, the testing device (also referred to as Antenna Under Test (AUT)) is placed in far field of a probe antenna (the probe antenna is an antenna which can transmit or receive power to/from the testing device, and which has a known radiation pattern and characteristics). In a far-field, the testing device's radiation pattern does not change shape with the distance between the testing device and the probe antenna.
In a near-space range, the testing device and the probe antenna are located close to each other.
A free-space range is a measurement location designed to simulate measurements that would be performed in space, i.e. where reflected waves from nearby objects and the ground are suppressed as much as possible. An anechoic chamber is an example of a free space range measurement location.
CATR is a facility that may be used to provide testing of antenna systems at frequencies where obtaining far-field spacing to the testing device may be difficult using traditional free space testing methods. In a CATR facility, a reflector is used to reflect the waves.
Antenna Reference Point (ARP) is a point located somewhere in the interface between the radio and the antenna of a base station. ARP is used as a reference in measurements and testing, and various parameters may be measured relative to the ARP. In AAS base station products, the access to the ARP will be limited or it will not be available. Hence, there will be no possibility to carry out conducted measurements found in conformance test requirements included in traditional specifications (e.g. TS 25.141, TS 36.141, TS 37.141 and TS 37.145-1). Also, all Radio Frequency (RF) testing at Research & Development (R&D) level is today done conducted at the ARP. For high frequencies, Over The Air (OTA) testing may be the only way of verifying RF characteristics, such as radiated transmit power (TS 37.145-2) and radiated unwanted emission (to be included in specifications).
OTA, which is an abbreviation for Over The Air, is a technology for transmitting radio signals over the air, as distinct from cable or wire transmission. Over The Air, is an interface that will be used to specify and verify highly integrated products where there possibly will be no ARP available and where the relevant performance need to be defined OTA, as opposed to for traditional radio base stations where performance is specified and verified in a conducted interface like ARP.
In the Third Generation Partnership Project (3GPP) Release-13 version of TS 37.145, a limited number of OTA requirements have been introduced (radiated transmit power and OTA sensitivity). There is an ambition to develop a specification in 3GPP with all RF characteristics defined in the radiated domain. This means that RF parameters needs to be tested in normal environmental conditions and some requirements are defined in extreme environmental conditions. Specific parameters, such as radiated transmit power, radiated unwanted emission, OTA sensitivity and frequency stability that today are measured using cable, will have to be measured OTA. When conducting OTA testing, absolute radiated power will correspond to Equivalent Isotropic Radiated Power (EIRP) and absolute received power will correspond to Equivalent Isotropic Sensitivity (EIS). Assuming a “black-box” approach without any knowledge about the test object implementation these two parameters will be verified in some sort of far field antenna test range, with validated measurement uncertainty assessment in normal condition environment
Also, there are regulatory requirements and/or specific customer requirements asking for that radiated transmit power, carrier frequency stability etc. should be verified OTA in extreme operating conditions. For example a normal operating condition has room temperature and no vibrations, and an extreme operating condition may have high or low temperature and substantial vibrations. The conditions applicable for normal operating condition and extreme operating conditions are defined by 3GPP, and will be described later.
Based on R&D quality assurance and customer requirement, the scope of extreme conditions could extend to also include vibration testing of RF characteristics. For receiver sensitivity there is no regulatory requirement to measure during extreme conditions, but it can be expected that customers will request such information.
The challenge with measuring absolute EIRP and absolute EIS OTA under extreme conditions is that EIRP and EIS are defined in the far-field region. The distance to the far-field region is determined of the physical size of the test object antenna aperture and the operating frequency. It is common that the far-field distance becomes very large, requiring large antenna test facilities. Also, relevant for all types of antenna test ranges is that they consist of high-precision mechanical equipment (such as positioners, reflectors, reference antennas, test range antennas) that are not designed for operation in large temperature ranges. Also, if the equipment could operate in extreme temperature condition, the amount of energy needed to cycle the test range would be enormous. Neither can it be made to vibrate according to what is required according to the environmental requirements.
To establish specifications with all RF requirements defined in the radiated domain, would put an impossible task on OTA test facility vendors to handle extreme conditions requirements.